Description

The pthread_mutexattr_setprotocol() and pthread_mutexattr_getprotocol() functions, respectively, set and get the protocol attribute
of a mutex attribute object pointed to by attr, which was previously
created by the pthread_mutexattr_init() function.

The protocol attribute defines the protocol to be followed in utilizing mutexes.
The value of protocol may be one of PTHREAD_PRIO_NONE, PTHREAD_PRIO_INHERIT, or
PTHREAD_PRIO_PROTECT, which are defined by the header <pthread.h>.

When a thread owns a mutex with the PTHREAD_PRIO_NONE protocol attribute,
its priority and scheduling are not affected by its mutex ownership.

When a thread is blocking higher priority threads because of owning one
or more mutexes with the PTHREAD_PRIO_INHERIT protocol attribute, it executes at
the higher of its priority or the priority of the highest priority
thread waiting on any of the mutexes owned by this thread and initialized
with this protocol.

When a thread owns one or more mutexes initialized with the PTHREAD_PRIO_PROTECT
protocol, it executes at the higher of its priority or the highest
of the priority ceilings of all the mutexes owned by this thread
and initialized with this attribute, regardless of whether other threads are blocked on
any of these mutexes.

While a thread is holding a mutex that has been initialized with
the PRIO_INHERIT or PRIO_PROTECT protocol attributes, it will not be subject
to being moved to the tail of the scheduling queue at its
priority in the event that its original priority is changed, such as by
a call to sched_setparam(). Likewise, when a thread unlocks a mutex that
has been initialized with the PRIO_INHERIT or PRIO_PROTECT protocol attributes, it will
not be subject to being moved to the tail of the scheduling queue
at its priority in the event that its original priority is changed.

If a thread simultaneously owns several mutexes initialized with different protocols, it
will execute at the highest of the priorities that it would have
obtained by each of these protocols.

If a thread makes a call to pthread_mutex_lock() for a mutex that
was initialized with the protocol attribute PTHREAD_PRIO_INHERIT, and if the calling thread
becomes blocked because the mutex is owned by another thread, then the
owner thread inherits the priority level of the calling thread for as
long as it continues to own the mutex. The implementation updates its
execution priority to the maximum of its assigned priority and all its
inherited priorities. Furthermore, if this owner thread becomes blocked on another mutex,
the same priority inheritance effect will be propagated to the other owner thread,
in a recursive manner.

If a thread with scheduling policy equal to SCHED_OTHER uses a mutex
initialized with the PTHREAD_PRIO_INHERIT or PTHREAD_PRIO_PROTECTprotocol attribute value, the effect on
the thread's scheduling and priority is unspecified.

The _POSIX_THREAD_PRIO_INHERIT and _POSIX_THREAD_PRIO_PROTECT options are designed to provide features to solve
priority inversion due to mutexes. A priority inheritance or priority ceiling mutex
is designed to minimize the dispatch latency of a high priority thread when
a low priority thread is holding a mutex required by the high
priority thread. This is a specific need for the realtime application
domain.

Threads created by realtime applications need to be such that their priorities
can influence their access to system resources (CPU resources, at least), in
competition with all threads running on the system.

Return Values

Upon successful completion, the pthread_mutexattr_getprotocol() and pthread_mutexattr_setprotocol() functions return 0. Otherwise,
an error number is returned to indicate the error.

Errors

The pthread_mutexattr_getprotocol() and pthread_mutexattr_setprotocol() functions will fail if:

EINVAL

The value specified by attr is NULL.

ENOSYS

Neither of the options _POSIX_THREAD_PRIO_PROTECT and _POSIX_THREAD_PRIO_INHERIT is defined and the system does not support the function.

ENOTSUP

The value specified by protocol is an unsupported value.

The pthread_mutexattr_getprotocol() and pthread_mutexattr_setprotocol() functions may fail if: